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bulletin: museum of comparative zoology. 



head as in many other invertebrates. The sperm head increases 

 rapidly in size by absorption from the surrounding egg cytoplasm, 

 and its chromatin is rearranged to form a large sperm nucleus. 

 Meanwhile the iirst polar body is discharged, followed in a few 

 minutes by the second. The chromatin remaining in the egg forms 

 an egg nucleus which fuses A^dth the sperm nucleus. At this time 

 both nuclei are of about the same size. The sperm asters degen- 

 erate, their centrosomes disappear, and in their places a pair of 

 minute cleavage asters arise. These enlarge to form the typical 

 cleavage asters with fibers extending throughout the substance of 

 the egg. It is still a matter of doubt as to whether the centrosomes 

 of the sperm asters persist and occupy the centers of the cleavage 



asters or whether they de- 

 generate, the centrosomes of 

 the cleavage asters being 

 entirely new foi-mations. 

 Kostanecki (:02), however, 

 considers the evidence in 

 favor of the former view 

 as almost conclusive. 



The segmentation of the 

 egg is total and equal as far 

 as the four-celled stage and 

 conforms throughout to the 

 regular spiral type. In the 

 eight-celled stage, however, 

 the four cells of the animal 

 pole are slightly, though 

 distinctly, larger than those 

 of the opposite side of the 

 egg.^ Such a peculiarity is 

 also characteristic of the 

 eggs of Cerehratidus lactens^ 

 and Micrura caeca^ although it is known in no other animals with a 

 spiral type of cleavage. 



1 The details of the process of cleavage and very interesting experiments on 

 the development of egg-fragments, isolated blastomeres, and blastulafrag- 

 ments are described in a recent paper by E. B. Wilson (:03) for Cerebratulus 

 lacteus. 



Fig. 30— Cerebratuliis marginatus. Pilidium 

 ten days after fertilization of tlie egg. Di- 

 gestive tract indicated l)y dotted lines. 



